1.Advanced Technology Research Institute, Laboratory of Infrared Materials and Devices, Ningbo University, Ningbo 315211, China 2.Key Laboratory of Photoelectric Materials and Devices of Zhejiang Province, Ningbo 315211, China
Fund Project:Project supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 61605094), the Key Program of the National Natural Science Foundation of China (Grant No. 61435009), and the K. C. Wong Magna Fund in Ningbo University, China.
Received Date:08 October 2018
Accepted Date:19 January 2019
Available Online:01 March 2019
Published Online:20 March 2019
Abstract:Microsphere lasers operating at the $2\;{\text{μ}}{\rm{m}}$ band have important applications in the fields of bio-medical sensing, laser radars, narrow linewidth optical filtering, and air-pollution monitoring. In this work, we utilize a novel type of chalcogenide glass, whose composition is Ge-Ga-Sb-S or 2S2G, to fabricate microsphere lasers. Compared with chalcogenide glasses used in previous microsphere lasers, this 2S2G glass is environmentally friendly. It also has a lower melting temperature and a higher characterization temperature, implying that 2S2G microspheres can be fabricated at lower temperatures and the crystallization problem happening in the sphere-forming process can be mitigated. A $\text{Tm}^{3+}\text{-}\text{Ho}^{3+} $ co-doping scheme is applied to the 2S2G glass, so that fluorescence light at ～$2\;{\text{μ}}{\rm{m}}$ can be obtained from the bulk glass. Owing to the superior properties of the 2S2G glass, we can utilize a droplet method to mass-produce hundreds of high-quality 2S2S microspheres in one experimental run. The diameters of microspheres fabricated in this work fall in a range of 50?$250\;{\text{μ}}{\rm{m}}$ and typical quality factors (Q factor) of microspheres are higher than 105. As a representative example, we characterize the optical properties of a $205.82\;{\text{μ}}{\rm{m}}$ diameter 2S2G microsphere. This microsphere is placed in contact with a silica fiber taper, so that the pump light can be evanescently introduced into the microsphere and the fluorescence light can be evanescently collected from the microsphere. A commercial laser diode (808 nm) is used as a pump source and an optical spectral analyzer is used to measure the transmission spectra of the microsphere/fiber taper coupling system. Apparent whispering gallery mode patterns in the ～$2\;{\text{μ}}{\rm{m}}$ band can be noted in the transmission spectra of the coupling system. When the pump power increases beyond a threshold of 0.848 mW, a lasing peak at 2080.54 nm can be obtained from the coupling system. Experimental results presented in this work show that this 2S2G chalcogenide glass is a promising base material for fabricating various active optical/photonic devices in the middle-wavelength and long-wavelength infrared spectra. Keywords:chalcogenide glass/ mid-infrared laser/ microsphere laser
从图1可见, 在Tm3+掺杂2S2G玻璃中存在两个主要的荧光峰, 分别位于$1.4\;{\text{μ}}{\rm{m}}$附近和$1.9\;{\text{μ}}{\rm{m}}$附近; 而在Tm3+-Ho3+共掺玻璃的荧光光谱则出现了一个位于$2\;{\text{μ}}{\rm{m}}$附近的荧光峰. 这是因为808 nm的抽运源首先促使了Tm3+离子实现了3H6→3H4跃迁, 而一部分位于3H4能级的Tm3+离子则出现了从3H4到3F4能级的非辐射多声子衰减. 这部分能量可转移到相邻的Ho3+离子并促使其实现5I7→5I8的能级跃迁, 并导致了1.8—2.1 ${\text{μ}}{\rm{m}}$波段荧光的产生. 为了采用漂浮粉末熔融法制备微球谐振腔, 首先将前面实验制得的块状2S2G玻璃研磨成粉末, 并倒入细孔筛中进行筛选. 保留粒径为0.048—0.300 mm的玻璃粉末, 混合均匀后放入超声波清洗机中反复清洗. 将粉末烘干后置入通有惰性保护气体的高温垂直加热炉具内. 玻璃粉末在高温下熔融, 并在表面张力作用下逐渐形成球形液滴, 并最终在收集冷却系统内形成固化形成玻璃微球. 本课题组自制的垂直加热炉具有可一次性批量制备数百枚质量良好的2S2G玻璃微球的能力. 图2展示了实验制备的一批典型的2G2S硫系玻璃微球. 图 2 实验制备的一批典型的2G2S硫系玻璃微球 Figure2. A typical batch of 2S2G microspheres fabricated in this work
利用显微镜观察实验所制备的微球并挑选出具有较好球形度和表面光洁度的微球用于后续的光学表征实验. 将挑选出的微球用紫外胶固定在切断后的石英光纤锥顶部, 并将固定有微球的光纤锥安装到三维纳米位移平台(Thorlabs, 型号: MAX381/M)上以实现对微球位置的精密控制. 为了测量微球的Q值, 采用一台可调谐激光器(EXFO,型号: FLS-2600)为抽运光源. 以一台红外探测器(Thorlabs, 型号: PDA20-CS-EC)测量微球/光纤锥耦合系统在各个波长的通光率. 图3展示了实验测得微球/光纤锥耦合系统位于1552.34 nm附近的一处典型吸收峰. 图中黑色圆点代表了实验数据, 红色实线代表了实验数据的高斯拟合. 图 3 位于1552.34 nm附近的一处典型吸收峰, 其中圆点代表实验数据实线则是高斯拟合曲线; 内插图是实验所选用的直径为$205.82\;{\text{μ}}{\rm{m}}$的2S2G玻璃微球 Figure3. A typical absorption valley at 1552.34 nm obtained with the microsphere/fiber taper coupling system. Dark spots and the red solid line represent experimental measurements and their Gaussian fit, respectively. The inset shows a microscopic image of the 2S2G microsphere used in this experiment, whose diameter is $205.82\;{\text{μ}}{\rm{m}}$
3.实验结果与分析为了测试微球/光纤锥耦合系统的荧光光谱, 使用一台808 nm半导体激光器(LEO, 型号: LE-LS-808)作为抽运光源, 耦合系统的输出端连接一台红外光谱分析仪(ANDO, 型号: AQ6317B)测量荧光光谱. 当抽运光功率未达到微球的激光阈值时, 在耦合系统的荧光光谱中可观测到明显的光学回廊模式. 图4展示了一颗直径为$205.82\;{\text{μ}}{\rm{m}}$微球在不同抽运功率下获得的光学回廊模. 图中黑色虚线为块状玻璃的荧光光谱. 图 4 直径为$205.82\;{\text{μ}}{\rm{m}}$的微球在1700—2150 nm波长范围内的光学回廊模, 其中黑色虚线表示块状玻璃的荧光光谱 Figure4. Whispering gallery modes within the wavelength span of 1700?2150 nm obtained from a $205.82\;{\text{μ}}{\rm{m}}$ diameter microsphere. The black dashed line represents the fluorescence spectrum of the bulk glass
从图4可以看出, 在微球基质材料中产生的荧光在微球谐振腔的模式选择作用下形成了明显包含周期分立光谱谐振峰的微球回廊模式. 这些谐振峰意味着与微球谐振腔本征模相符的光波模式场得到了共振增强, 从而在微球中形成了光学回廊模. 从图4也可以看出, 回廊模的包络形状和块状玻璃的荧光光谱是匹配的. 从图4还可以看出, 随着抽运功率的逐渐增强, 回廊模的整体强度也逐步增强. 当抽运功率达到0.848 mW后, 可以在2080.54 nm附近观察到明显的激光峰. 并且随着抽运功率增加, 激光的峰值功率也会随之增大. 图5展示了抽运功率与激光峰值功率的关系. 图5的插图对比了抽运功率为0.782 mW (黑色曲线)和0.848 mW (红色曲线)时耦合系统的荧光光谱. 我们可以看到当抽运功率从0.782 mW增加到0.848 mW后, 耦合系统的荧光光谱上开始出现单纵模激光模式. 图 5 微球激光功率与抽运功率的关系(插图为抽运功率为0.782 mW和0.848 mW时耦合系统的荧光光谱) Figure5. Relationship between the microsphere laser power and the pump power. Inset: fluorescence spectra obtained when the pump power are 0.782 mW (black curve) and 0.848 mW (red curve)
图 1 Tm3+离子单掺(黑色曲线)和Tm3+-Ho3+离子共掺(红色曲线) 2G2S玻璃荧光光谱对比图
图 2 实验制备的一批典型的2G2S硫系玻璃微球
图 3 位于1552.34 nm附近的一处典型吸收峰, 其中圆点代表实验数据实线则是高斯拟合曲线; 内插图是实验所选用的直径为
图 4 直径为
图 5 微球激光功率与抽运功率的关系(插图为抽运功率为0.782 mW和0.848 mW时耦合系统的荧光光谱)
图 6 2S2G微球激光器的多模输出光谱(包含两个激光峰)
